how do i create a link/button that changes something in a textarea?

like when the editor here on the forum, the buttons for bold, italics and even the smilies...

something like that.

With a lot of javascript

And it's a real PITA to write one, believe me!

But there is FCKeditor

FCKeditor is a really great product... it's fairly easy to setup, and easy to customize (although you don't really need to for a lot of applications).